Sensors that detect and measure the conductivity of fluids are useful for a variety of purposes. U.S. patent application Ser. No. 11/402,0062 by Guest, for example, offers a novel method and apparatus for controlling electrolytic processes purely through such conductivity measurements, rather than by the more traditional measurement of the pH or oxidation reduction potential of the electrolyzed product. While that invention offers a simpler and more reliable way to control such processes, a simpler—and thus implicitly more reliable—conductivity sensor would improve that innovation further, and be of great benefit in a variety of other applications, as well. Current sensors require complex circuitry and measurements, and are subject to degradation by the accumulation of deposits on sensor surfaces; also, when used to measure the conductivity of materials with low conductivity—such as water—they can see very low voltages at the receiver coil, resulting in inaccurate measurements overwhelmed by background “noise” from stray electromagnetic waves in the environment and similar. This invention has none of these drawbacks.